Process of manufacturing stainless steel



United States Patent Oil 3,168,398 PROCESS OF MANUFACTURING STAINLESS STEEL Kenkichi Tachiki, 117 Sanya-cho, Meguro-lru, and Masami Sata, 17 Z-chome, Kaminakazato, Kita-ku, both of Tokyo, Japan No Drawing. Filed Dec. 6, 1960, Ser. No. 73,996 Claims priority, application Japan, Dec. 14, 1959, S t/39,223 8 Claims. (Cl. 75-1305) The present invention relates to a process of manu facturing an improved stainless steel which is very tough and anticorrosive.

The conventional stainless steel consists of steel containing high precentage of nickel or chromium.

The stainless steel, however, cannot be manufactured at low cost because of high cost of said nickel or chromium.

Therefore, an object of the present invention is to provide a process of manufacturing an improved stainless steel having the same properties as the conventional stainless steel of high class, without using nickel or by using a small amount of nickel. 7

Another object of the present invention is to provide an improved stainless steel of high class.

The above mentioned object and other objects of this invention have been attained by melting a mixture of iron, chromium and calcium at a temperature above 1400 C. in an inert gas to obtain an alloy of said mixture. In said process, iron and chromium may be mixed in the state of an alloy and an amount of carbon may be mixed, if necessary, to said mixture.

We have succeeded already in manufacturing an anticorrosive alloy by heating the mixture of iron, chromium and calcium or of iron-chromium alloy and calcium at a temperature of 800 C. to 1300 C. Recently, however, we have discovered that when said mixture is heated at a temperature above 1400 C. so as to make said mixture melt completely, the contained calcium decreases, chemical-analytically, up to Generally, when such a metal having a relatively low melting point as calcium is to be comelted with a metal having a high melting point, the former metal evaporates out before formation of a complete eutectic mixture, or even when a complete eutectic mixture has been formed, said former metal evaporates out in the end, so that almost none of said former metal remains. Accordingly, it has been deemed that calcium cannot remain in the bath of an eutectic mixture of iron and chromium or of iron-chromium alloy. However, we have discovered and confirmed by application of electron ray diffraction that the calcium remains in the alloy in a completely different state due to its conversion. Perhaps the converted constituent may be a new element ranking between calcium and strontium and the molecular weight of said element is near that of zinc. Consequently, the stainless steel of this invention is deemed to be a conventional chrome steel which is a two-element alloy consisting of iron and chromium, but, in actuality, it is three-element alloy containing a new element confirmed by electron ray diffraction. The blending rate of the new element, although it cannot be confirmed, may be deemed to be almost equal to the blending rate of the blended calcium.

The blending rates of iron and chromium for obtaining the alloy according to the process of this invention are, respectively, 30-90% and 1050%. If necessary, carbon may be alloyed with the said alloy. However, it is a very particular case to alloy up to 6% of carbon. So as to make such high carbon alloy, 15% carbon is mixed in. Accordingly, the product of this invention should be generally deemed as a cast iron, but, in practice, it has, remarkably, the various properties of the normal steel.

The carbon is not contained in the free state but in the state of carbide which is very stable. This fact differs from the case of the nickel-chrome steel. The corrosion appearing between the particles which occurs in the case of nickel-chrome steel is very minor in the stainless steel of this invention.

In embodying this invention, iron of the kind usable for manufacturing normal steel, and 97% chromium prepared by the thermite-process are used, but in the case of using iron-chrome alloy, chromium and carbon of 5% are used. In these cases, calcium of 99% quality is used and some amount of carbon powder is mixed in for adjusting the amount of the carbon.

Prior to alloying the mixture of the materials, the mixing materials are previouslyl pulverized.

In the following, the actual steps of this invention will be described. In the case of using iron and chromium, the chromium is blended with the iron, and to this mixture is added calcium. Then said mixture is heated at a temperature between 1400 C. and 1700 C. until said mixture is melted uniformly. When iron-chrome alloy is used, calcium is first added to said alloy and then this mixture is heated at the same temperature as the abovestated range until the uniformly fused mixture is obtained. Melting is carried out in an alumina crucible arranged in an electric furnace such as a high frequency electric furnace or tanman furnace. In this case, the crucible is put in an argon atmosphere.

The alloy obtained by the above-mentioned process has a luster and an anticorrosive property more pronounced than those of the stainless steel of (18-8) designation (18% Cr,8% Ni).

Referring to the alloy obtained, according to this invention, by using 2-20% of calcium, 10-50% of chromium, and 30-90% or iron, the amount of loss of said alloy due to corrosion caused by hydrochloric acid, sulfuric acid, and nitric acid, each being 10% concentration, are as, follows:

Loss amount (mg./cm. /hr.)

(1) Hydrochloric acid (10%) 0.3 (2) Sulfuric acid (10%) 0.6 (3) Nitric acid (10%) 0.25

The above-stated alloy, as will be understood from the above table, is very excellent in the anticorrosive property in comparison with (l88) stainless steel. The Brinell hardness of the above alloy varies within the wide range between 130 and 450. Generally, however, the alloy having (Brinell hardness) of about 350 is used in practice. The alloy manufactured by this invention is very tough and may be ranked in the class of the most excellent nickel-chrome steel of the third kind. For example, the impact strength, yield point and tensile strength of the most tough alloy, are, respectively, 13 kg. m./mm. kg./mm. and kg./mm. Since the alloy manufactured by this invention can be elongated about 15%, is readily rolled and otherwise worked. Moreover, the alloy manufactured by this invention has a characteristic that its remelting and casting are possible.

Example 1 Percent Calcium 0.01 Chromium 29.3

Patented Feb. 2, 1965 cheeses This product did not dissolve in hydrochloric acid or 10% sulfuric acid and its surface lustrewas slightly reduced by concentrated nitric acid. The important physical properties of saidproduct were as follows:

Hardness (Brinell) 160 Impact strength kg. m./mm. 10

Yield point "kg/mm?" 80 Tensile strength "kg/mm?" 115 Elongation "percent; 14

Example 2 In an alumina crucible arranged in a high frequency furnace, 65 gr. of iron powder prepared from normal steel, 15 gr. of 97% chromium, and gr. of 99% calcium were thoroughly mixed by stirring and this mixture was 7 heated for a relatively long period of time at about 800 C. while argon gas 'was'passcd through said crucible. Then the temperature'of said mixture was raised up to 1700 C., whereby said mixture was completely melted. The analysed compositionof the final product taken out from the crucible after cooling thereof was as follows:

. Percent Calcium 0.01 Chromium 17.5

The physical properties of the product of this example were the same as those of the product of Example 1.

Example 3 Percent Calcium 0.01 Chromium 21.7 Carbon 1.2

The acid proof character of said product was more pronounced than that of the products of Examples 1 and 2, and the Brinell hardness of said product was 210.

Example 4 In the place of iron of 65 gr. in Example 1, a mixture of 61 gr. of iron and 4 gr. of carbon powder was used,

Percent Calcium 0.03 Chromium 29.2

Carbon 3.9

The acid proof character of the product of this example was most pronounced and its Brinell hardness was 330, which is the highest among those of the products of the other examples.

What we claim is:

1. A process for making stainless steel comprising mixing 10-50% finely divided chromium, 220% calcium, up to 6% carbon, the balance being iron and then heating said mixture to a temperature of between 1400 C. and 1700 C. in an inert gas atmosphere.

2. The process of claim 1 wherein the gas is argon.

3. A'process for manufacturing stainless steel comprising mixing 30-90% finely divided iron, 1050% finely divided chromium, 225% finely, divided calcium, and up to 15% finely divided carbon and heating said mixture to a temperature higher than 1400 C. in an inert atmosphere.

4. The process of claim 3 wherein the inert atmosphere is a vacuum.

5. A process of manufacturing stainless steel comprising mixing finely divided iron-chrome alloy, finely divided calcium and finely divided carbon, the iron-chrome alloy is in such proportions as to represent iron in a quantity of 30-90% and the chromium in a quantity of 10-50%, the calcium being in a quantity of 2-25% and the carbon being in a quantity of up to 15% and then heating said mixture to a temperature higher than 1400 C. in an inert atmosphere.

' 6. The process of claim 5 wherein the inert atmosphere is a vacuum.

7. The process of claim 1 wherein the iron and chromium are present in an alloy thereof.

8. The process of claim 2 wherein the iron and chromium are present in an alloy thereof.

References Cited in the file of this patent UNITED STATES PATENTS 2,144,200 Rohn et al Jan. 17, 1939 FOREIGN PATENTS 3,275/31 Australia July 30, 1931 

1. A PROCESS FOR MAKING STAINLESS STEEL COMPRISING MIXING 10-50% FINELY DIVIDED CHROMIUM, 2-20% CALCIUM, UP TO 6% CARBON, THE BALANCE BEING IRON AND THEN HEATING SAID MIXTURE TO A TEMPERATURE OF BETWEEN 1400*C. AND 1700*C.IN AN INERT GAS ATMOSPHERE. 